1. Field of the Invention
The present invention relates to continuous flow drilling systems and methods.
2. Description of the Related Art
In many drilling operations in drilling in the earth to recover hydrocarbons, a drill string made by assembling pieces or joints of drill tubulars or pipe with threaded connections and having a drill bit at the bottom is rotated to move the drill bit. Typically drilling fluid, such as oil or water based mud, is circulated to and through the drill bit to lubricate and cool the bit and to facilitate the removal of cuttings from the wellbore that is being formed. The drilling fluid and cuttings returns to the surface via an annulus formed between the drill string and the wellbore. At the surface, the cuttings are removed from the drilling fluid and the drilling fluid is recycled.
As the drill bit penetrates into the earth and the wellbore is lengthened, more joints of drill pipe are added to the drill string. This involves stopping the drilling while the tubulars are added. The process is reversed when the drill string is removed or tripped, e.g. to replace the drilling bit or to perform other wellbore operations. Interruption of drilling may mean that the circulation of the mud stops and has to be re-started when drilling resumes. This can be time consuming, can cause deleterious effects on the walls of the wellbore being drilled, and can lead to formation damage and problems in maintaining an open wellbore. Also, a particular mud weight may be chosen to provide a static head relating to the ambient pressure at the top of a drill string when it is open while tubulars are being added or removed. The weighting of the mud can be very expensive.
To convey drilled cuttings away from a drill bit and up and out of a wellbore being drilled, the cuttings are maintained in suspension in the drilling fluid. If the flow of fluid with cuttings suspended in it ceases, the cuttings tend to fall within the fluid. This is inhibited by using relatively viscous drilling fluid; but thicker fluids require more power to pump. Further, restarting fluid circulation following a cessation of circulation may result in the overpressuring of a formation in which the wellbore is being formed.
FIG. 1 is a prior art diagrammatic view of a portion of a continuous flow system. FIG. 1A is a sectional elevation of a portion of the union used to connect two sections of drill pipe, showing a short nipple to which is secured a valve assembly. FIG. 1B is a sectional view taken along the line 1B-1B of FIG. 1A.
A derrick 1 supports long sections of drill pipe 8 to be lowered and raised through a tackle having a lower block 2 supporting a swivel hook 3. The upper section of the drill string 8 includes a tube or Kelly 4, square or hexagonal in cross section. The Kelly 4 is adapted to be lowered through a square or hexagonal hole in a rotary table 5 so, when the rotary table is rotated, the Kelly will be rotated. To the upper end of the Kelly 4 is secured a connection 6 by a swivel joint 7. The drill pipe 8 is connected to the Kelly 4 by an assembly which includes a short nipple 10 which is secured to the upper end of the drill pipe 8, a valve assembly 9, and a short nipple 25 which is directly connected to the Kelly 4. A similar short nipple 25 is connected to the lower end of each section of the drill pipe.
Each valve assembly 9 is provided with a valve 12, such as a flapper, and a threaded opening 13. The flapper 12 is hinged to rotate around the pivot 14. The flapper 12 is biased to cover the opening 13 but may pivot to the dotted line position of FIG. 1A to cover opening 15 which communicates with the drill pipe or Kelly through short a nipple 25 into the screw threads 16. The flapper 12 is provided with a screw threaded extension 28 which is adapted to project into the threaded opening 13. A plug member 27 is adapted to be screwed on extension 28 as shown in FIG. 1A, normally holding the valve 12 in the position covering the side opening in the valve assembly. Normally, before drilling commences, lengths of drill pipe are assembled in the vicinity of the drill hole to form “stands” of drill pipe. Each stand may include two or more joints of pipe, depending upon the height of the derrick, length of the Kelly, type of drilling, and the like. The sections of the stand are joined to one another by a threaded connection, which may include nipples 25 and 10, screwed into each other. At the top of each stand, a valve assembly 9 is placed. It will be observed that the valve body acts as a connecting medium or union between the Kelly and the drill string.
Normally, oil well fluid circulation is maintained by pumping drilling fluid from the sump 11 through pipe 17 through which the pump 18 takes suction. The pump 18 discharges through a header 39 into valve controlled flexible conduit 19 which is normally connected to the member 6 at the top of the Kelly, as shown in FIG. 1. The mud passes down through the drill pipe assembly out through the openings in the drill bit 20, into the wellbore 21 where it flows upwardly through the annulus and is taken out of the well casing 22 through a pipe 23 and is discharged into the sump 11. The Kelly 4, during drilling, is being operated by the rotary table 5. When the drilling has progressed to such an extent that is necessary to add a new stand of drill pipe, the tackle is operated to lift the drill string so that the last section of the drill pipe and the union assembly composed of short nipple 25, valve assembly 9, and short nipple 10 are above the rotary table. The drill string is then supported by engaging a spider (not shown).
The plug 27 is unscrewed from the valve body and a hose 29, which is controlled by a suitable valve, is screwed into the screw threaded opening 13. While this operation takes place, the circulation is being maintained through hose 19. When connection is made, the valve controlling hose 29 is opened and momentarily mud is being supplied through both hoses 19 and 29. The valve controlling hose 19 is then closed and circulation takes place as before through hose 29. The Kelly is then disconnected and a new stand is joined to the top of the valve body, connected by screw threads 16. After the additional stand has been connected, the valve controlling hose 19 is again opened and momentarily mud is being circulated through both hoses 19 and 29. Then the valve controlling hose 29 is closed, which permits the valve 12 to again cover opening 13. The hose 29 is then disconnected and the plug 27 is replaced.